1. Field of the Invention
The present invention relates generally to devices for electrically simulating fluid pumping and compressor systems, and more specifically to a system for simulating a gas compressor and associated pipelines.
2. Description of Related Art
Installation or modification of natural gas or other fluid distribution systems requires consideration of a number of factors before work is undertaken. Variations in loads, distribution paths, pipe sizes and compressor speeds all have effects on the operation of the system as a whole. Compression waves created in the gas by the operation of reciprocating pumps and compressors are especially troublesome, as fluid acoustic resonances can be set up in the system. These resonances increase metal fatigue and shorten the life of joints, valves and other components of the system, and often degrade compressor performance. Acoustic resonances through centrifugal pumps can cause surge and loss of pressure head.
To assist in planning for control of pulsations and vibrations, an electrical analog of all fluid handling components can be created. Present electrical systems analogize current to mass flow of the gas and voltage to pressure. Inductors, capacitors and resistors are used to model the acoustical properties of pipes and other components in the distribution system. A detailed model of pumping or compressing system or subsystem can be set up and studied to predict the effects caused by changing various parameters in its operation.
In order to utilize easily obtained components, the operating frequency of the electrical analog is typically substantially higher than that of the mechanical system. An electrical to mechanical frequency ratio is typically in the neighborhood of 1000 to one. Component values and analog system parameters are chosen so that all events which occur during the operation of the model reflect events which will take place in the mechanical system. For example, the presence of an electrical resonance in the analog system at a certain frequency corresponds to an acoustical resonance in the mechanical system at the corresponding mechanical speed.
The present state of the art in pumping system analogs is typified by U.S. Pat. No. 2,951,638, issued to Hughes, et al. The system described therein shows a model of a reciprocating compressor utilizing a capacitor which is driven by a sinusoidal voltage source. Due to inaccuracies inherent in the use of a fixed capacitor to model the changing volume of a compressor cylinder, the driving voltage signal to the capacitor must be shaped to compensate for both pressure and volume changes.
Since a fixed capacitance is used to model a time varying volume, it is not possible for the model to present the correct acoustic compliant reactance to both the intake and discharge ports of the model when the respective valves are opened to the remainder of the circuit. The volume of the mechanical cylinder is at or near a minimum value near the end of the discharge cycle, and at or near a maximum value near the end of the intake cycle. Thus, it is at best possible to only approximate the proper impedance to either the intake or discharge port of the model cylinder, with the impedance at the other port differing substantially from the correct value.
Further, it is necessary that the correct current, which is analogous to mass flow of the gas, be transported through the analog on each cycle. In order to meet this criteria, it is frequently impossible to present the proper cylinder impedance to either the intake or discharge port.
In practice, it is necessary to arbitrarily choose a capacitance which lies somewhere in the range between the minimum and maximum values presented to the cylinder port. The magnitude and shape of the driving signal are then aribitrarily adjusted until the proper pressure-volume diagram is obtained for the particular cylinder being modeled, and the value of the capacitance is then arbitrarily readjusted to obtain the proper analog current flow. The analog models the mechanical compressor only approximately, with a degree of accuracy primarily dependent upon the skill of the model operator. The capacitor can in no sense be considered to correspond to any real physical volume, with the result that the reactance presented by the model cylinder to the rest of the circuit is incorrect. This inaccuracy causes the results of the simulation to be correspondingly inaccurate.
Additionally, because the driving signal for the electrical model has been arbitrarily shaped, conventional phase meters cannot be used to control the relative phasing between several cylinders, which is necessary when modeling a multi-cylinder compressor. Accurate phasing between cylinders is this rendered difficult.
Many pumping and transport systems utilize centrifugal, as well as reciprocating, compressors. The pressure differential across the compressor and the pump volume vary with changes in the instantaneous operating conditions of the compressor. Variations in intake and/or discharge pressures of flow volume, caused for example by reciprocating compressors and acoustic resonances in the piping, change the centrifugal compressor pressure flow outputs in accordance with specifications supplied by the manufacturers. Centrifugal compressors present an acoustic impedance to the pumping system which must be taken into account to accurately model the system.
Present analog systems use diodes to simulate unidirectional mechanical valves. Because of the inherent characteristics of diodes, they do not accurately simulate the acoustic properties of such valves. Additionally, there is not suitable method for simulating the operation of a pressure relief valve.
It would be desirable for an electrical analog of gas pumping system to accurately simulate the acoustic properties of valves, reciprocating and centrifugal, compressors, cylinders and other mechanical objects in the system. It would also be desirable that such simulation includes suitable means for monitoring operation of the analog so that accurate determinations of the analog, and thus the mechanical system, operating conditions can be made.
It is therefore an object of the present invention to provide an electrical model of a gas pumping system, which utilizes reciprocating or centrifugal pumps or compressors, which accurately simulates the acoustic properties of the various mechanical parts.
It is another object of the present invention to provide such a system simulation which provides suitable monitoring devices so that an operator can insure accurate operation of the simulation and determine previously unknown values corresponding to mechanical conditions.